I've had this problem for a while on two computers, and through a few versions. I have also created a new profile on one (as the old profile was for some reason locking up every time I used it, and thankfully I use IMAP, so it was easy to recreate). I have sub folders on my mail server (currently delivered by Exim and Dovecot for the IMAP folder structure) and when I expand the sub folders to select one I am unable to select the top one until I have selected another one. Clicking on it does nothing, but clicking on another and seeing the messages in there, then clicking on it shows the messages. This only happens with the top folder, so I'm wondering if the code is somehow classing that as already selected. I thought I'd ask here before looking into raising a bug report.